src/core/ext/filters/client_channel/resolving_lb_policy.cc

   1 /*
   2  *
   3  * Copyright 2015 gRPC authors.
   4  *
   5  * Licensed under the Apache License, Version 2.0 (the "License");
   6  * you may not use this file except in compliance with the License.
   7  * You may obtain a copy of the License at
   8  *
   9  *     http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing, software
  12  * distributed under the License is distributed on an "AS IS" BASIS,
  13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14  * See the License for the specific language governing permissions and
  15  * limitations under the License.
  16  *
  17  */
  18
  19 #include <grpc/support/port_platform.h>
  20
  21 #include "src/core/ext/filters/client_channel/resolving_lb_policy.h"
  22
  23 #include <inttypes.h>
  24 #include <limits.h>
  25 #include <stdbool.h>
  26 #include <stdio.h>
  27 #include <string.h>
  28
  29 #include <grpc/support/alloc.h>
  30 #include <grpc/support/log.h>
  31 #include <grpc/support/string_util.h>
  32 #include <grpc/support/sync.h>
  33
  34 #include "src/core/ext/filters/client_channel/backup_poller.h"
  35 #include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
  36 #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
  37 #include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
  38 #include "src/core/ext/filters/client_channel/resolver_registry.h"
  39 #include "src/core/ext/filters/client_channel/retry_throttle.h"
  40 #include "src/core/ext/filters/client_channel/server_address.h"
  41 #include "src/core/ext/filters/client_channel/service_config.h"
  42 #include "src/core/ext/filters/client_channel/subchannel.h"
  43 #include "src/core/ext/filters/deadline/deadline_filter.h"
  44 #include "src/core/lib/backoff/backoff.h"
  45 #include "src/core/lib/channel/channel_args.h"
  46 #include "src/core/lib/channel/connected_channel.h"
  47 #include "src/core/lib/channel/status_util.h"
  48 #include "src/core/lib/gpr/string.h"
  49 #include "src/core/lib/gprpp/inlined_vector.h"
  50 #include "src/core/lib/gprpp/manual_constructor.h"
  51 #include "src/core/lib/gprpp/sync.h"
  52 #include "src/core/lib/iomgr/combiner.h"
  53 #include "src/core/lib/iomgr/iomgr.h"
  54 #include "src/core/lib/iomgr/polling_entity.h"
  55 #include "src/core/lib/profiling/timers.h"
  56 #include "src/core/lib/slice/slice_internal.h"
  57 #include "src/core/lib/slice/slice_string_helpers.h"
  58 #include "src/core/lib/surface/channel.h"
  59 #include "src/core/lib/transport/connectivity_state.h"
  60 #include "src/core/lib/transport/error_utils.h"
  61 #include "src/core/lib/transport/metadata.h"
  62 #include "src/core/lib/transport/metadata_batch.h"
  63 #include "src/core/lib/transport/static_metadata.h"
  64 #include "src/core/lib/transport/status_metadata.h"
  65
  66 namespace grpc_core {
  67
  68 //
  69 // ResolvingLoadBalancingPolicy::ResolverResultHandler
  70 //
  71
  72 class ResolvingLoadBalancingPolicy::ResolverResultHandler
  73     : public Resolver::ResultHandler {
  74  public:
  75   explicit ResolverResultHandler(
  76       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
  77       : parent_(std::move(parent)) {}
  78
  79   ~ResolverResultHandler() {
  80     if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
  81       gpr_log(GPR_INFO, "resolving_lb=%p: resolver shutdown complete",
  82               parent_.get());
  83     }
  84   }
  85
  86   void ReturnResult(Resolver::Result result) override {
  87     parent_->OnResolverResultChangedLocked(std::move(result));
  88   }
  89
  90   void ReturnError(grpc_error* error) override {
  91     parent_->OnResolverError(error);
  92   }
  93
  94  private:
  95   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
  96 };
  97
  98 //
  99 // ResolvingLoadBalancingPolicy::ResolvingControlHelper
 100 //
 101
 102 class ResolvingLoadBalancingPolicy::ResolvingControlHelper
 103     : public LoadBalancingPolicy::ChannelControlHelper {
 104  public:
 105   explicit ResolvingControlHelper(
 106       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
 107       : parent_(std::move(parent)) {}
 108
 109   RefCountedPtr<SubchannelInterface> CreateSubchannel(
 110       const grpc_channel_args& args) override {
 111     if (parent_->resolver_ == nullptr) return nullptr;  // Shutting down.
 112     if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
 113     return parent_->channel_control_helper()->CreateSubchannel(args);
 114   }
 115
 116   grpc_channel* CreateChannel(const char* target,
 117                               const grpc_channel_args& args) override {
 118     if (parent_->resolver_ == nullptr) return nullptr;  // Shutting down.
 119     if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
 120     return parent_->channel_control_helper()->CreateChannel(target, args);
 121   }
 122
 123   void UpdateState(grpc_connectivity_state state,
 124                    UniquePtr<SubchannelPicker> picker) override {
 125     if (parent_->resolver_ == nullptr) return;  // Shutting down.
 126     // If this request is from the pending child policy, ignore it until
 127     // it reports READY, at which point we swap it into place.
 128     if (CalledByPendingChild()) {
 129       if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
 130         gpr_log(GPR_INFO,
 131                 "resolving_lb=%p helper=%p: pending child policy %p reports "
 132                 "state=%s",
 133                 parent_.get(), this, child_,
 134                 grpc_connectivity_state_name(state));
 135       }
 136       if (state != GRPC_CHANNEL_READY) return;
 137       grpc_pollset_set_del_pollset_set(
 138           parent_->lb_policy_->interested_parties(),
 139           parent_->interested_parties());
 140       parent_->lb_policy_ = std::move(parent_->pending_lb_policy_);
 141     } else if (!CalledByCurrentChild()) {
 142       // This request is from an outdated child, so ignore it.
 143       return;
 144     }
 145     parent_->channel_control_helper()->UpdateState(state, std::move(picker));
 146   }
 147
 148   void RequestReresolution() override {
 149     // If there is a pending child policy, ignore re-resolution requests
 150     // from the current child policy (or any outdated child).
 151     if (parent_->pending_lb_policy_ != nullptr && !CalledByPendingChild()) {
 152       return;
 153     }
 154     if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
 155       gpr_log(GPR_INFO, "resolving_lb=%p: started name re-resolving",
 156               parent_.get());
 157     }
 158     if (parent_->resolver_ != nullptr) {
 159       parent_->resolver_->RequestReresolutionLocked();
 160     }
 161   }
 162
 163   void set_child(LoadBalancingPolicy* child) { child_ = child; }
 164
 165  private:
 166   bool CalledByPendingChild() const {
 167     GPR_ASSERT(child_ != nullptr);
 168     return child_ == parent_->pending_lb_policy_.get();
 169   }
 170
 171   bool CalledByCurrentChild() const {
 172     GPR_ASSERT(child_ != nullptr);
 173     return child_ == parent_->lb_policy_.get();
 174   };
 175
 176   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
 177   LoadBalancingPolicy* child_ = nullptr;
 178 };
 179
 180 //
 181 // ResolvingLoadBalancingPolicy
 182 //
 183
 184 ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
 185     Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
 186     UniquePtr<char> child_policy_name,
 187     RefCountedPtr<LoadBalancingPolicy::Config> child_lb_config,
 188     grpc_error** error)
 189     : LoadBalancingPolicy(std::move(args)),
 190       tracer_(tracer),
 191       target_uri_(std::move(target_uri)),
 192       child_policy_name_(std::move(child_policy_name)),
 193       child_lb_config_(std::move(child_lb_config)) {
 194   GPR_ASSERT(child_policy_name_ != nullptr);
 195   // Don't fetch service config, since this ctor is for use in nested LB
 196   // policies, not at the top level, and we only fetch the service
 197   // config at the top level.
 198   grpc_arg arg = grpc_channel_arg_integer_create(
 199       const_cast<char*>(GRPC_ARG_SERVICE_CONFIG_DISABLE_RESOLUTION), 0);
 200   grpc_channel_args* new_args =
 201       grpc_channel_args_copy_and_add(args.args, &arg, 1);
 202   *error = Init(*new_args);
 203   grpc_channel_args_destroy(new_args);
 204 }
 205
 206 ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
 207     Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
 208     ProcessResolverResultCallback process_resolver_result,
 209     void* process_resolver_result_user_data, grpc_error** error)
 210     : LoadBalancingPolicy(std::move(args)),
 211       tracer_(tracer),
 212       target_uri_(std::move(target_uri)),
 213       process_resolver_result_(process_resolver_result),
 214       process_resolver_result_user_data_(process_resolver_result_user_data) {
 215   GPR_ASSERT(process_resolver_result != nullptr);
 216   *error = Init(*args.args);
 217 }
 218
 219 grpc_error* ResolvingLoadBalancingPolicy::Init(const grpc_channel_args& args) {
 220   resolver_ = ResolverRegistry::CreateResolver(
 221       target_uri_.get(), &args, interested_parties(), combiner(),
 222       UniquePtr<Resolver::ResultHandler>(New<ResolverResultHandler>(Ref())));
 223   if (resolver_ == nullptr) {
 224     return GRPC_ERROR_CREATE_FROM_STATIC_STRING("resolver creation failed");
 225   }
 226   // Return our picker to the channel.
 227   channel_control_helper()->UpdateState(
 228       GRPC_CHANNEL_IDLE, UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
 229   return GRPC_ERROR_NONE;
 230 }
 231
 232 ResolvingLoadBalancingPolicy::~ResolvingLoadBalancingPolicy() {
 233   GPR_ASSERT(resolver_ == nullptr);
 234   GPR_ASSERT(lb_policy_ == nullptr);
 235 }
 236
 237 void ResolvingLoadBalancingPolicy::ShutdownLocked() {
 238   if (resolver_ != nullptr) {
 239     resolver_.reset();
 240     if (lb_policy_ != nullptr) {
 241       if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 242         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down lb_policy=%p", this,
 243                 lb_policy_.get());
 244       }
 245       grpc_pollset_set_del_pollset_set(lb_policy_->interested_parties(),
 246                                        interested_parties());
 247       lb_policy_.reset();
 248     }
 249     if (pending_lb_policy_ != nullptr) {
 250       if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 251         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down pending lb_policy=%p",
 252                 this, pending_lb_policy_.get());
 253       }
 254       grpc_pollset_set_del_pollset_set(pending_lb_policy_->interested_parties(),
 255                                        interested_parties());
 256       pending_lb_policy_.reset();
 257     }
 258   }
 259 }
 260
 261 void ResolvingLoadBalancingPolicy::ExitIdleLocked() {
 262   if (lb_policy_ != nullptr) {
 263     lb_policy_->ExitIdleLocked();
 264     if (pending_lb_policy_ != nullptr) pending_lb_policy_->ExitIdleLocked();
 265   } else {
 266     if (!started_resolving_ && resolver_ != nullptr) {
 267       StartResolvingLocked();
 268     }
 269   }
 270 }
 271
 272 void ResolvingLoadBalancingPolicy::ResetBackoffLocked() {
 273   if (resolver_ != nullptr) {
 274     resolver_->ResetBackoffLocked();
 275     resolver_->RequestReresolutionLocked();
 276   }
 277   if (lb_policy_ != nullptr) lb_policy_->ResetBackoffLocked();
 278   if (pending_lb_policy_ != nullptr) pending_lb_policy_->ResetBackoffLocked();
 279 }
 280
 281 void ResolvingLoadBalancingPolicy::StartResolvingLocked() {
 282   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 283     gpr_log(GPR_INFO, "resolving_lb=%p: starting name resolution", this);
 284   }
 285   GPR_ASSERT(!started_resolving_);
 286   started_resolving_ = true;
 287   channel_control_helper()->UpdateState(
 288       GRPC_CHANNEL_CONNECTING,
 289       UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
 290   resolver_->StartLocked();
 291 }
 292
 293 void ResolvingLoadBalancingPolicy::OnResolverError(grpc_error* error) {
 294   if (resolver_ == nullptr) {
 295     GRPC_ERROR_UNREF(error);
 296     return;
 297   }
 298   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 299     gpr_log(GPR_INFO, "resolving_lb=%p: resolver transient failure: %s", this,
 300             grpc_error_string(error));
 301   }
 302   // If we already have an LB policy from a previous resolution
 303   // result, then we continue to let it set the connectivity state.
 304   // Otherwise, we go into TRANSIENT_FAILURE.
 305   if (lb_policy_ == nullptr) {
 306     grpc_error* state_error = GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
 307         "Resolver transient failure", &error, 1);
 308     channel_control_helper()->UpdateState(
 309         GRPC_CHANNEL_TRANSIENT_FAILURE,
 310         UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(state_error)));
 311   }
 312   GRPC_ERROR_UNREF(error);
 313 }
 314
 315 void ResolvingLoadBalancingPolicy::CreateOrUpdateLbPolicyLocked(
 316     const char* lb_policy_name,
 317     RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config,
 318     Resolver::Result result, TraceStringVector* trace_strings) {
 319   // If the child policy name changes, we need to create a new child
 320   // policy.  When this happens, we leave child_policy_ as-is and store
 321   // the new child policy in pending_child_policy_.  Once the new child
 322   // policy transitions into state READY, we swap it into child_policy_,
 323   // replacing the original child policy.  So pending_child_policy_ is
 324   // non-null only between when we apply an update that changes the child
 325   // policy name and when the new child reports state READY.
 326   //
 327   // Updates can arrive at any point during this transition.  We always
 328   // apply updates relative to the most recently created child policy,
 329   // even if the most recent one is still in pending_child_policy_.  This
 330   // is true both when applying the updates to an existing child policy
 331   // and when determining whether we need to create a new policy.
 332   //
 333   // As a result of this, there are several cases to consider here:
 334   //
 335   // 1. We have no existing child policy (i.e., we have started up but
 336   //    have not yet received a serverlist from the balancer or gone
 337   //    into fallback mode; in this case, both child_policy_ and
 338   //    pending_child_policy_ are null).  In this case, we create a
 339   //    new child policy and store it in child_policy_.
 340   //
 341   // 2. We have an existing child policy and have no pending child policy
 342   //    from a previous update (i.e., either there has not been a
 343   //    previous update that changed the policy name, or we have already
 344   //    finished swapping in the new policy; in this case, child_policy_
 345   //    is non-null but pending_child_policy_ is null).  In this case:
 346   //    a. If child_policy_->name() equals child_policy_name, then we
 347   //       update the existing child policy.
 348   //    b. If child_policy_->name() does not equal child_policy_name,
 349   //       we create a new policy.  The policy will be stored in
 350   //       pending_child_policy_ and will later be swapped into
 351   //       child_policy_ by the helper when the new child transitions
 352   //       into state READY.
 353   //
 354   // 3. We have an existing child policy and have a pending child policy
 355   //    from a previous update (i.e., a previous update set
 356   //    pending_child_policy_ as per case 2b above and that policy has
 357   //    not yet transitioned into state READY and been swapped into
 358   //    child_policy_; in this case, both child_policy_ and
 359   //    pending_child_policy_ are non-null).  In this case:
 360   //    a. If pending_child_policy_->name() equals child_policy_name,
 361   //       then we update the existing pending child policy.
 362   //    b. If pending_child_policy->name() does not equal
 363   //       child_policy_name, then we create a new policy.  The new
 364   //       policy is stored in pending_child_policy_ (replacing the one
 365   //       that was there before, which will be immediately shut down)
 366   //       and will later be swapped into child_policy_ by the helper
 367   //       when the new child transitions into state READY.
 368   const bool create_policy =
 369       // case 1
 370       lb_policy_ == nullptr ||
 371       // case 2b
 372       (pending_lb_policy_ == nullptr &&
 373        strcmp(lb_policy_->name(), lb_policy_name) != 0) ||
 374       // case 3b
 375       (pending_lb_policy_ != nullptr &&
 376        strcmp(pending_lb_policy_->name(), lb_policy_name) != 0);
 377   LoadBalancingPolicy* policy_to_update = nullptr;
 378   if (create_policy) {
 379     // Cases 1, 2b, and 3b: create a new child policy.
 380     // If lb_policy_ is null, we set it (case 1), else we set
 381     // pending_lb_policy_ (cases 2b and 3b).
 382     if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 383       gpr_log(GPR_INFO, "resolving_lb=%p: Creating new %schild policy %s", this,
 384               lb_policy_ == nullptr ? "" : "pending ", lb_policy_name);
 385     }
 386     auto& lb_policy = lb_policy_ == nullptr ? lb_policy_ : pending_lb_policy_;
 387     lb_policy =
 388         CreateLbPolicyLocked(lb_policy_name, *result.args, trace_strings);
 389     policy_to_update = lb_policy.get();
 390   } else {
 391     // Cases 2a and 3a: update an existing policy.
 392     // If we have a pending child policy, send the update to the pending
 393     // policy (case 3a), else send it to the current policy (case 2a).
 394     policy_to_update = pending_lb_policy_ != nullptr ? pending_lb_policy_.get()
 395                                                      : lb_policy_.get();
 396   }
 397   GPR_ASSERT(policy_to_update != nullptr);
 398   // Update the policy.
 399   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 400     gpr_log(GPR_INFO, "resolving_lb=%p: Updating %schild policy %p", this,
 401             policy_to_update == pending_lb_policy_.get() ? "pending " : "",
 402             policy_to_update);
 403   }
 404   UpdateArgs update_args;
 405   update_args.addresses = std::move(result.addresses);
 406   update_args.config = std::move(lb_policy_config);
 407   // TODO(roth): Once channel args is converted to C++, use std::move() here.
 408   update_args.args = result.args;
 409   result.args = nullptr;
 410   policy_to_update->UpdateLocked(std::move(update_args));
 411 }
 412
 413 // Creates a new LB policy.
 414 // Updates trace_strings to indicate what was done.
 415 OrphanablePtr<LoadBalancingPolicy>
 416 ResolvingLoadBalancingPolicy::CreateLbPolicyLocked(
 417     const char* lb_policy_name, const grpc_channel_args& args,
 418     TraceStringVector* trace_strings) {
 419   ResolvingControlHelper* helper = New<ResolvingControlHelper>(Ref());
 420   LoadBalancingPolicy::Args lb_policy_args;
 421   lb_policy_args.combiner = combiner();
 422   lb_policy_args.channel_control_helper =
 423       UniquePtr<ChannelControlHelper>(helper);
 424   lb_policy_args.args = &args;
 425   OrphanablePtr<LoadBalancingPolicy> lb_policy =
 426       LoadBalancingPolicyRegistry::CreateLoadBalancingPolicy(
 427           lb_policy_name, std::move(lb_policy_args));
 428   if (GPR_UNLIKELY(lb_policy == nullptr)) {
 429     gpr_log(GPR_ERROR, "could not create LB policy \"%s\"", lb_policy_name);
 430     char* str;
 431     gpr_asprintf(&str, "Could not create LB policy \"%s\"", lb_policy_name);
 432     trace_strings->push_back(str);
 433     return nullptr;
 434   }
 435   helper->set_child(lb_policy.get());
 436   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 437     gpr_log(GPR_INFO, "resolving_lb=%p: created new LB policy \"%s\" (%p)",
 438             this, lb_policy_name, lb_policy.get());
 439   }
 440   char* str;
 441   gpr_asprintf(&str, "Created new LB policy \"%s\"", lb_policy_name);
 442   trace_strings->push_back(str);
 443   grpc_pollset_set_add_pollset_set(lb_policy->interested_parties(),
 444                                    interested_parties());
 445   return lb_policy;
 446 }
 447
 448 void ResolvingLoadBalancingPolicy::MaybeAddTraceMessagesForAddressChangesLocked(
 449     bool resolution_contains_addresses, TraceStringVector* trace_strings) {
 450   if (!resolution_contains_addresses &&
 451       previous_resolution_contained_addresses_) {
 452     trace_strings->push_back(gpr_strdup("Address list became empty"));
 453   } else if (resolution_contains_addresses &&
 454              !previous_resolution_contained_addresses_) {
 455     trace_strings->push_back(gpr_strdup("Address list became non-empty"));
 456   }
 457   previous_resolution_contained_addresses_ = resolution_contains_addresses;
 458 }
 459
 460 void ResolvingLoadBalancingPolicy::ConcatenateAndAddChannelTraceLocked(
 461     TraceStringVector* trace_strings) const {
 462   if (!trace_strings->empty()) {
 463     gpr_strvec v;
 464     gpr_strvec_init(&v);
 465     gpr_strvec_add(&v, gpr_strdup("Resolution event: "));
 466     bool is_first = 1;
 467     for (size_t i = 0; i < trace_strings->size(); ++i) {
 468       if (!is_first) gpr_strvec_add(&v, gpr_strdup(", "));
 469       is_first = false;
 470       gpr_strvec_add(&v, (*trace_strings)[i]);
 471     }
 472     size_t len = 0;
 473     UniquePtr<char> message(gpr_strvec_flatten(&v, &len));
 474     channel_control_helper()->AddTraceEvent(ChannelControlHelper::TRACE_INFO,
 475                                             message.get());
 476     gpr_strvec_destroy(&v);
 477   }
 478 }
 479
 480 void ResolvingLoadBalancingPolicy::OnResolverResultChangedLocked(
 481     Resolver::Result result) {
 482   // Handle race conditions.
 483   if (resolver_ == nullptr) return;
 484   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
 485     gpr_log(GPR_INFO, "resolving_lb=%p: got resolver result", this);
 486   }
 487   // We only want to trace the address resolution in the follow cases:
 488   // (a) Address resolution resulted in service config change.
 489   // (b) Address resolution that causes number of backends to go from
 490   //     zero to non-zero.
 491   // (c) Address resolution that causes number of backends to go from
 492   //     non-zero to zero.
 493   // (d) Address resolution that causes a new LB policy to be created.
 494   //
 495   // We track a list of strings to eventually be concatenated and traced.
 496   TraceStringVector trace_strings;
 497   const bool resolution_contains_addresses = result.addresses.size() > 0;
 498   // Process the resolver result.
 499   const char* lb_policy_name = nullptr;
 500   RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config;
 501   bool service_config_changed = false;
 502   char* service_config_error_string = nullptr;
 503   if (process_resolver_result_ != nullptr) {
 504     grpc_error* service_config_error = GRPC_ERROR_NONE;
 505     service_config_changed = process_resolver_result_(
 506         process_resolver_result_user_data_, result, &lb_policy_name,
 507         &lb_policy_config, &service_config_error);
 508     if (service_config_error != GRPC_ERROR_NONE) {
 509       service_config_error_string =
 510           gpr_strdup(grpc_error_string(service_config_error));
 511       if (lb_policy_name == nullptr) {
 512         // Use an empty lb_policy_name as an indicator that we received an
 513         // invalid service config and we don't have a fallback service config.
 514         OnResolverError(service_config_error);
 515       } else {
 516         GRPC_ERROR_UNREF(service_config_error);
 517       }
 518     }
 519   } else {
 520     lb_policy_name = child_policy_name_.get();
 521     lb_policy_config = child_lb_config_;
 522   }
 523   if (lb_policy_name != nullptr) {
 524     // Create or update LB policy, as needed.
 525     CreateOrUpdateLbPolicyLocked(lb_policy_name, lb_policy_config,
 526                                  std::move(result), &trace_strings);
 527   }
 528   // Add channel trace event.
 529   if (service_config_changed) {
 530     // TODO(ncteisen): might be worth somehow including a snippet of the
 531     // config in the trace, at the risk of bloating the trace logs.
 532     trace_strings.push_back(gpr_strdup("Service config changed"));
 533   }
 534   if (service_config_error_string != nullptr) {
 535     trace_strings.push_back(service_config_error_string);
 536     service_config_error_string = nullptr;
 537   }
 538   MaybeAddTraceMessagesForAddressChangesLocked(resolution_contains_addresses,
 539                                                &trace_strings);
 540   ConcatenateAndAddChannelTraceLocked(&trace_strings);
 541 }
 542
 543 }  // namespace grpc_core